This invention relates to lightning detectors and particularly to detectors which are relatively simple and easy to install, yet are resistant to false triggering by man made electrical noise.
The destructive power of lightning is well known, at best a lightning strike is disruptive of normal activities, at worst a lightning strike can cause severe property damage, personal injury and death. The ability to reliably detect lightning discharges at a distance, so that a warning can be sounded or preventative action undertaken has been desired for many years. Lightning discharges are accompanied by electrostatic and electromagnetic fields which can be detected at distance. Accordingly many systems for lightning detection detect these electromagnetic phenomenon. The problem with lightning detectors that use the detection of the electrostatic and electromagnetic phenomena to detect a lightning discharge is not that they don't detect lightning, they do, the problem is they detect many other man made electrical noise and falsely interpret the noise as a lightning discharge. A wide variety of appliances and equipment used in the home and industry are capable of generating electrical noise that can falsely trigger a lightning detector. Electric motors, solenoids, welders, florescent lighting ballasts and stove ignitors are only some of the devices that can provide the requisite electromagnetic discharge to falsely trigger a lightning detector.
Electrical circuitry capable of detecting lightning discharges is shown in U.S. Pat. Nos. 4,823,115 and 4,912,459 to McCallie and U.S. Pat. No. 5,083,042 to Merchant. However, the circuits shown in these patents while capable of lightning detection contain no effective means of preventing false triggering. One solution to this problem has been to recommend that the detector be remotely mounted outdoors and not be connected to house current (through which electrical noise is transmitted) which adds to cost and complexity and does not fully prevent the device from falsing. Another solution to false detection is through the use of multiple electrical and magnetic antennas whose signals are combined and processed to provide accurate lightning detection. Such multiple detectors are shown in U.S. Pat. Nos. 4,115,737; 4,198,599 and 4,806,851 to Krider et al. However the circuitry needed to process multiple detectors is complex and removes such detectors from consideration for use in the home. In U.S. Pat. No. 5,448,161 a photodiode used to detect a lightning flash is used in conjunction with an electromagnetic detector to accurately detect a lightning discharge. However this is also relatively complex and requires that the photodiode be mounted so that it has an unobstructed sky view, which is not possible in many locations. An extremely complex lightning detection system suitable for use in aircraft is shown in U.S. Pat. No. 5,305,210 and related patents. This system uses digital signal processing to analyze input lightning waveforms in a lightning warning. Such an approach is of course is far too expensive for general commercial or home applications.
The present invention provides apparatus and methodology for lightning detection that uses relatively simple and hence reliable circuitry. A device following the teachings of the present invention does not respond to common household and industrial noise. Despite the fact the most electrical noise that can falsely trigger lightning detectors is carried by the electrical lines, the present apparatus can be powered by household power. Apparatus constructed in accordance with the present invention does not require remotely mounted antennas or multiple antennas. This enables the present device to be constructed as a single unit thus reducing expense. The present device is capable of adjusting its own sensitivity when distant lightning discharges have been detected and is capable of triggering multiple levels of precautionary steps as lightning approaches.
The present invention includes apparatus and methodology for detection of lightning and is resistant to man made electrical noise. The invention includes an antenna and amplifier for receiving the electrostatic and/or electromagnetic discharges of lightning and electrical noise and outputs an analog electrical signal representative of the discharge. The analog signal is input to a threshold detection circuitry whose output is activated when the input signal rises above a predetermined level. The output of the threshold circuitry is a series of pulses of varying length. A computer or other programmable device is connected to the output of the threshold circuitry and opens an event window when the first pulse is received. The number of pulses received and the total duration of the pulses is determined during the event window. The number and total duration of the pulses is compared to stored pulse signatures of lightning discharges and man made electrical noise. If the pulse signature correlates with that of lightning an alert signal is generated, if the signature correlates with that of electrical noise no alert is generated and the event window is reset. The generation of a first alert may be used to change the length of the event window or other parameters to change sensitivity and/or to generate various stages of alerts. In addition to the generation of an alert, the apparatus may be used to trigger a precautionary action such as the raising of lightning rods by equipment such as that shown in our U.S. Pat. No. 5,657,197 issued Aug. 12, 1997.